Non-metallic coatings in the form of oxide ceramic layers applied on the surfaces of metallic components are widely used in modern engineering applications, typically when the components are required to have a high wear resistance or corrosion resistance. Non-metallic coatings also show significant prospects in emerging high-technology applications.
As an example, alumina-based ceramic surface layers formed on aluminium or aluminium alloy components, provide protection to and enhance the functionality of the components. This is due to the excellent physical and chemical properties of alumina, such as high hardness, high electrical resistivity and chemical stability. Such surface layers are widely used as wear-resistant and corrosion-resistant coatings in mechanical components (especially for moving parts experiencing high contact loads and strains), insulating coatings in electrical and electronic engineering, decorative coatings in construction, and as chemically inert coatings in chemical engineering applications.
Oxide ceramic coatings can be formed on metal substrates by a number of different methods. For example, coatings may be formed by deposition from precursor oxides, by brushing, spraying, or condensation from a vapour or liquid phase. Coatings may also be formed by thermal or electrochemical conversion of a portion of the surface of the metal substrate into an oxide.
Deposition coating techniques allow the use of a wide range of oxide materials but do not always provide good coating adhesion, uniformity and surface finish.
Conversion techniques provide better adhesion, but the range of oxide materials available as a coating is limited by the composition of parent metal.
Thermally activated methods of conversion are unsuitable for treatment of hardened and low-melting-point metal substrates, which are preferentially coated by electrochemical techniques. Relevant electrochemical conversion methods are based on anodic oxidation of the metal surface in aqueous electrolytes and are categorised into conventional low-voltage anodising and high-voltage plasma-assisted processes, such as plasma electrolytic oxidation (PEO). PEO is also known as microarc or microplasmic oxidation or spark anodising. These electrochemical methods can be used for treatment of a wide range of materials, including valve metals (e.g. Mg, Al, Ti, Zr, Nb and Ta) as well as semi-metals and semiconductors (e.g. Si, Ge and GaAs).
Anodising is the most common and versatile electrochemical conversion technique, and can be easily scaled up and automated so that up to 100 m2 of surface area can be processed simultaneously. The method used to anodise a component generally includes the steps of (i) immersion of the component in a tank containing an electrolytic solution and equipped with a counter electrode; (ii) application of a potential difference between the component and the counter electrode to produce electric current across the electrolyte and (iii) maintaining the potential difference over a period of time to obtain a desirable thickness of the oxide layer.
Both acidic and alkaline electrolytic solutions (electrolytes) are used for anodising, however genuine oxides can be formed only in the former. In alkaline solutions, anodising results in the formation of porous or gel-like hydroxide deposits (as described in U.S. Pat. No. 7,780,838), with poor protective and functional properties. The use of some acid based processes (e.g. chromic acid anodising) is currently limited by the law due to hazardous nature of the components involved.
During anodising, an oxide layer is formed as a result of the following anodic electrochemical processes:At the oxide-electrolyte interface: H2OO2−+2H+  (1)At the metal-oxide interface: AlAl3++3e−  (2)Net reaction: 2Al+3H2OAl2O3+6H++6e−  (3)
The contribution of oxy-anions to the electrochemical process is negligible, with their incorporation into anodic film structure occurring mainly due to adsorption. Consequently, there is limited scope to control the film chemical composition by varying the oxy-anion content, and any performance enhancement can only be achieved via additional post-anodising treatments. Such treatments include sealing and impregnation for anti-corrosion and tribological performance, colouring and dyeing for optical and aesthetic appearance, and loading with metals for catalytic performance and manufacturing of nano-wires.
Importantly, protons released according to reaction (1) cause local electrolyte acidification in the vicinity of the anode, which increases the risk of oxide dissolution. This risk rises dramatically if the metal substrate is heterogeneous, or if the surface is contaminated. Careful preparation and cleaning of the metal component prior to anodising is therefore essential.
When exposed to an electrolyte having a pH of less than 4, Al2O3 becomes chemically unstable, even on clean and homogeneous surfaces. This constrains the maximum allowed current density to between 3 and 5 A/dm2, which in turn limits the film growth rate and imposes strict requirements on electrolyte composition and temperature control. These issues may be addressed by introduction of electrolyte cooling, rigorous circulation of electrolyte and pauses in anodic polarisation, e.g. by the application of pulsed or AC current modes. Thus, refrigeration down to between 0 and 5° C. is commonly used to produce anodic alumina films with thickness up to between 30 and 50 μm and hardness of between 500 and 600 HV.
U.S. Pat. No. 7,776,198 describes a method for anodizing objects in an electrolyte flow by using current pulses of variable magnitude, whereas I. De Graeve, et al [Electrochim. Acta, 52 (2006) 1127-1134] report on studies of AC anodising of aluminium in solutions of sulphuric and phosphoric acids. Generic disadvantages of these techniques consist in reduced coating growth rate and anodising efficiency compared to DC modes, which is due to the fact that neither pause nor cathodic polarisation contribute to the oxide film formation.
Anodic films have an amorphous physical structure. Films thicker than 1 μm are heterogeneous, featuring thin (0.1 to 0.3 μm) inner barrier layers and a thick porous outer layer comprising ordered honeycomb cells. While these structures may be useful for the production of free-standing ceramic membranes and nanofabrication, the in-plane mechanical properties of anodised surfaces are compromised, which affects their tribological performance as well as the component's bulk strength.
Thus, although scalable and versatile, anodising is environmentally unfriendly technology that requires careful surface preparation and offers a limited protection to the metal components, unless they are subjected to complex post-treatment and finishing procedures. The functional performance of anodic films is also limited due to an inability to control their chemical and phase composition.
Attempts to address major issues associated with anodising resulted in the development of several plasma-assisted electrochemical oxidation processes; these are unified here by the generic term of PEO. Unlike anodising, PEO is carried out in alkaline electrolytes wherein the process at the metal-electrolyte interface does not yield protons:2OH−H2O+O2−  (4)
As the local pH cannot drop below 7, there is no risk of oxide chemical dissolution, hence no need for laborious surface preparation and cleaning. Oxide growth is hindered, however, by the formation of aluminium hydroxide:Al3++3OH−→Al(OH)3  (5)
The overall technological procedure for PEO is similar to that of anodising. One of the main differences is that the applied voltage magnitude is much higher (200 to 800 V). This high applied voltage triggers electrical breakdowns of the growing oxide film. As a result numerous localised micro-discharge events occur on the surface and are manifested in localised flashes of light (also sometimes termed ‘microspark discharge’, or ‘microarcs’). During each individual micro-discharge event, a thin conductive channel is rapidly developed and extinguished, providing a short circuit path for charge and mass transfer across the film. The temperature in the core of this channel is estimated to rise up to between 5 and 20×103 K in a matter of microseconds. This promotes development of localised plasma bulbs where plasma-assisted chemical reactions take place between components of the metal substrate and the electrolyte. Adjacent to discharge surface regions, porous and gel-like hydroxide deposits previously formed according to reaction (5) are dehydrated, sintered and re-crystallised. High-temperature and mixed oxide (e.g. spinel) phases comprising both metal and electrolyte species can be produced in this way as part of a macroscopically dense ceramic surface layer up to 200 to 300 μm thick.
Compounds synthesised by micro-discharge events can exhibit high hardness and dielectric strength (e.g. α-Al2O3 coatings produced by PEO have a hardness of between 18 and 25 GPa and a dielectric strength of between 20 and 40 kV/mm), good chemical inertness (e.g. alumina, silica) and low thermal conductivity (e.g. PEO alumina coating has been reported to have a thermal conductivity of between 0.8 and 1.7 W/mK [J A Curran and T W Clyne, The thermal conductivity of plasma electrolytic oxide coatings on aluminium and magnesium, Surf. Coat. Technol., 199 (2005) 177-183]). Thus, PEO coatings are attractive for many protective applications.
There are drawbacks to the use of PEO coatings. High thermal gradients associated with discharge events inevitably lead to the generation of internal stresses that can cause cracks in the ceramic layer. These cracks detrimentally affect both wear and corrosion resistance and should be avoided. Also, plasma-chemical reaction products ejected from discharge channels and quenched by the electrolyte tend to form coarse porous structures (the average pore size can be as high as tens of microns) with high surface roughness. The coarse outer layer can make up to 60% of the total coating thickness and must be removed if the component is intended to work in mechanical contact with other surfaces. A dense inner layer starts forming only when the coating thickness exceeds a certain threshold (typically 20 to 30 μm); thinner coatings are rather uneven and offer limited protection.
PEO technology is excessively energy intense, yet laborious and expensive post-treatment finishing is still required. The surface chemistry and phase composition can be controlled within a relatively wide range, however, the coatings are usually produced at a low rate (between 0.5 and 2 μm/min) and develop a course uneven structure with cracks and other defects that compromise their performance.
Both anodising and PEO techniques have limitations. It is an aim of the present invention to provide improved methods of forming a non-metallic coating on a metal or semi-metal substrate. It is a further aim to provide non-metallic coatings with improved properties compared to anodised or PEO generated coatings.